The present invention relates to a gas turbine power plant, and more particularly to a power plant having a state-of-the-art steam turbine and a state-of-the-art gas turbine in which the gas turbine is driven by a mixture of gases and steam.
Typically, gas turbine power plants are powered by the combustion of clean fuels such as distillate oil or natural gas, which combustion products are passed through the gas turbine to drive the turbine. At rated conditions, such combustion products of clean fuels may reach temperatures in the range of about 1730.degree.-2120.degree. F. and gas pressures of up to 7 to 12 atmospheres prior to being expanded through the turbine. The metals in the state-of-the-art gas turbines are able to withstand the corrosive and erosive action of the combustion products of clean fuels as well as these temperature levels. Accordingly, such simple cycle gas turbines are able to reach efficiencies of approximately 30%, despite high levels of temperature and heat energy carried away in the exhaust gases. Unit ratings tend to be restricted, because of these difficult design requirements, but ratings from a few hundred kilowatts to over 100 megawatts are in commercial service.
Steam driven power plants are also used. They can be built economically for a larger range of ratings and in larger sizes than simple cycle gas turbines. Steam temperatures as high as 1000.degree. F.-1050.degree. F., with pressures of up to several hundred atmospheres are common. Overall plant efficiency levels of 33-36% have been achieved (despite the relatively lower maximum steam temperatures) because very little of the theoretically available energy is left in the power plant discharge streams of stack gas (250.degree.-300.degree. F.) or condenser cooling water (85.degree.-100.degree. F.). The steam generating furnaces for steam power plants have successfully burned a wide range of low-grade, unclean fuels such as solid coal, coke, lignite, industrial wastes; lower grade liquid fuels such as residual oils and tars; as well as premium "clean" fuels such as natural gas, when the latter was justified by favorable fuel prices, fuel availability, and the lower costs for pollution control. However, the thermal efficiency of these steam power plants is not as high when the lower grade fuels are used because of their more difficult combustion control conditions and the energy requirements of the pollution control system.
Each system (simple cycle gas turbines or steam turbines) has been utilized depending on the needs of the particular electric utility or industrial system. In the recent past, shortages of the clean fuels (distillate oil or natural gas) has caused their price to increase more rapidly than that of solid fuels (coal, wood, wastes, etc.). The United States, while it has limited remaining deposits of such clean fuels, has vast quantities of the "less clean" solid fuels such as fossil fuels, including coal, lignite, and peat, and solid renewable fuels including wood, biomass, agricultural waste, municipal waste and industrial waste. Many of these solid fuels are characterized by relatively low energy density, high bulk density, high levels of impurities and, in the case of the renewable fuels, a large fuel collection radius per unit of power generated. The latter application favors many relatively small plants rather than a much larger plant, of course. Accordingly, economic pressure has been exerted on the power plant industry to develop economical power plants which can utilize such low grade solid fuels in small to moderately sized plants at an efficiency which approaches that attainable in gas turbine and steam turbine power plants when they are operated with clean fuels.
When such solid fuels are used in a simple gas turbine cycle or combined gas turbine cycle (gas turbine thermodynamically coupled in any of several well-known arrangements with a steam turbine), they must first be gasified, liquified or reduced to minute dimensions suitable for combustion in solid form in direct air suspension. If significant quantities of erosive or corrosive species exist in the combustion products, they must then be cleaned, before being passed to the turbine. Alternatively, the solid fuels can be burned in an external combustion chamber utilizing heat exchangers to transfer the thermal energy of the combustion gases to a separate clean gas stream so as to exclude corrosive and combustion products which, if passed directly to the gas turbine, would dramatically shorten its life.
Willyoung, U.S. Pat. No. 4,116,005, discloses a system in which gas turbine motive gases are indirectly heated by the combustion of "dirty" (solid) fuels and are utilized to drive a gas turbine that is thermodynamically coupled to a steam turbine. A thermodynamically coupled dual gas turbine/steam turbine (combined cycle) system is utilized to recover thermal energy from the gas turbine exhaust stream to increase the overall efficiency of the power plant. This is desirable since the efficiency of a power plant employing metallic heat exchangers to supply thermal energy to gas turbines tends to be limited because the heat exchangers can only heat the motive gases passing therethrough to temperatures of up to about 1500.degree. F. As pointed out earlier, present state-of-the-art gas turbines can utilize motive gases at temperatures of up to about 2100.degree. F. to obtain high levels of power output and efficiency. When a gas at a lower temperature is passed therethrough, the power and efficiency of the gas turbine are decreased. Accordingly, while the efficiency of the system of Willyoung, U.S. Pat. No. 4,116,005, was an improvement over earlier art, permitting the direct use of solid fuel forms, it did not fully utilize the state-of-the-art capability of the gas turbines.
A different variation of a combined cycle turbine system powered by the burning of solid fuels is found in Willyoung, U.S. Pat. No. 4,223,529 where improvements are described which reduce the combustor size. However, this system again utilized a gas turbine which is driven by gases at less than the present day's state-of-the-art maximum temperature. Thus, again the full capability and efficiency potential of the gas turbines is not achieved.
These limitations are avoided in another power plant to be found in Willyoung, U.S. Pat. No. 4,253,300, in which the gas turbine is driven by "clean" compressed gases. First, there is heated the "clean" gases with lower grade "dirty" fuels in an external heat exchanger to the allowable temperature limits of that heat exchanger. After the compressed gases have passed through and have been heated in the heat exchanger, they are combined with a clean hydrocarbon fuel and the mixture is combusted to bring the motive combustion gas stream to the state-of-the-art temperature capability of the gas turbine, about 1800.degree.-2100.degree. F. This resulting hot gas stream is then expanded through the gas turbine. This system, while resulting in a more effective utilization of the capability of a state-of-the-art gas turbine system, nevertheless draws only a base part of its energy from the combustion of lower-cost, low-quality solid fuels. The remainder of its output, even though obtained at very high incremental efficiency, comes from the burning of more costly "clean" fuel (either naturally obtained or derived by the refining or conversion of "dirtier" natural fuel forms).
There is also the teaching of Kydd, et al, U.S. Pat. No. 3,693,347, which discloses the use of controlling means to optimize injection of steam into combustion gases to drive a gas turbine. The turbine of this system is operated by the burning of clean hydrocarbon fuels and it does not disclose the use of indirect heating of the motive gas stream to drive the gas turbine. Further, there is the teaching of Chang, U.S. Pat. No. 3,978,661, which discloses a turbine which can utilize both steam and gas. However, this disclosure is of a specially designed turbine and does not set forth the utilization or operation of a state-of-the-art gas turbine system.
Accordingly, it is one object of the present invention to provide for a state-of-the-art power plant in which the heat from solid or low quality fuels is utilized both to heat indirectly a motive stream composed of a mixture of steam and gases to drive a a gas turbine and also to generate essentially pure steam which is utilized to power a steam turbine.
It is an additional object of the present invention to provide an improved power plant using state-of-the-art apparatus in which combustion gases formed by the burning of wood or coal are utilized indirectly to heat a motive gaseous stream mixture composed of steam and air to drive a gas turbine and also to form steam to drive a steam turbine.
It is still a further object of the present invention to provide for a low-cost improved power plant utilizing essentially the full capability of state-of-the-art equipment in which solid fuels are burned to form dirt-laden combustion gases in which lower temperature thermal energy from such combustion gases is used indirectly to form steam, said steam being utilized to drive a steam turbine, and wherein spent steam from the steam turbine is mixed with a motive gaseous stream which, in turn, is indirectly heated by higher temperature thermal energy from said combustion gases and then utilized to drive a state-of-the-art gas turbine.
It is yet another object of the present invention to provide an improved gas turbine combined cycle power plant which can utilize "dirty" solid fuel for its primary energy source, and which utilizes state-of-the-art equipment in which a gas turbine is powered by a mixture of steam and air and wherein the overall efficiency of the plant is increased substantially by utilizing the thermal energy in the combustion gas stream just prior to exhaust to produce steam, which steam is used to drive an integrated steam turbine.
It is yet a further object of the present invention to provide a low-cost method for operating an improved gas turbine power plant in which the gas turbine is driven by a gaseous mixture of steam and air which is indirectly heated by the burning of solid fuel, and wherein the efficiency of the overall power plant can reach or exceed about 30%, depending on the moisture content, and hence the stack gas latent heat losses, of the fuel.